In a staged combustion gas turbine engine, burners of the engine are divided into at least two groups which can, for convenience, be referred to as pilot burners and main burners. It is to be understood that the main burner group may comprise a number of main burner sub-groups, but this distinction is not of importance to the invention. Generally, while an engine is operative, fuel is supplied continuously to the pilot burners, but is supplied to the main burners only while the thrust and/or emission requirements imposed upon the engine require that the main burners are operative in addition to the pilot burners.
In the accompanying drawings FIG. 1 is a diagrammatic representation of a known arrangement of a fuel metering unit supplying metered fuel to a conventional fuel staging unit which in turn distributes the fuel to pilot burners and main burners of an engine. In FIG. 1 the fuel metering unit (FMU) 11 and the fuel staging unit (FSU) 12 are shown in broken line boundaries and output lines 13, 14 from the FSU are connected in use to the pilot burner manifold and the main burner manifold respectively of the associated gas turbine engine. Fuel from a reservoir is supplied at low pressure to the inlet of a fixed displacement pump, for example a gear pump 15, driven by the main shaft of the gas turbine engine so as to be operated, and thus produce an output, directly related to the speed of rotation of the engine shaft. The output of the pump 15 is supplied through conventional filtering into the FMU 11.
As will be well understood by those skilled in the art the following description of a conventional FMU and FSU is restricted to basic components of those units. In practice both the FMU and FSU may well incorporate additional components effecting refinements in the operation of the units.
High pressure fuel (HP) from the pump 15 is directed within the FMU 11 to the inlet of a fuel metering valve 16 which supplies metered fuel at pressure PX to the inlet of a pressure raising and shut-off valve (PRSOV) 17. The output of the PRSOV 17 is supplied through a line 18 to the FSU 12. Within the FMU 11 a pressure drop control and spill valve (PDSV) 19 senses the pressure drop across the metering valve 16 (HP-PX) and spills excess delivery flow from HP to LP in order to maintain a constant pressure drop across the metering orifice of the valve 16 as the metering orifice is opened and closed in use. The PRSOV 17 establishes a minimum fuel pressure in the FMU upstream of the PRSOV 17 below which the PRSOV 17 isolates the FMU from the FSU by preventing flow into the line 18. Furthermore, the PRSOV 17 can be operated in response to predetermined command signals to close irrespective of the fuel pressure upstream the PRSOV to isolate the FSU from the FMU and so shut-down the associated engine. FIG. 1 illustrates a normal shut-off servo valve (SOSV) 21 and a turbine overspeed shut-off servo valve (TOSSV) 22 associated with the PRSOV 17. The SOSV 21 receives shut-off signals from the conventional electronic engine control unit (EECU) (not shown) of the engine control system to cause the SOSV 21 to initiate closure of the PRSOV 17 when appropriate command signals are issued for example by the pilot. The TOSSV 22 on the other hand is usually independent of the EECU and receives its signals from a monitoring system monitoring the speed of rotation of the shaft of the engine and initiating closure of the PRSOV 17 to shut-down the engine when the monitored engine speed exceeds a predetermined safe value.
The position of the metering spool, and thus the flow number of the metering orifice, of the metering valve 16 is controlled by a metering valve servo valve (MVSV) 23 forming part of the metering valve 16 and receiving fuel demand signals from the EECU of the system and positioning the valve spool accordingly. A position sensor 24 conveniently in the form of a linear variable differential transformer (LVDT) monitors the position of the spool of the metering valve 16 and supplies spool position signals back to the EECU.
The FSU 12 receives fuel metered by the FMU 11 by way of the line 18 to the inlet of a staging valve (SV) 25. The staging valve 25 can divide the flow from the line 18 into a pilot burner flow and a main burner flow in the lines 13 and 14 respectively. The valve 25 incorporates a staging valve servo valve (SVSV) 26 and a sensor 27 in the form of an LVDT for controlling and monitoring the position of the spool of the staging valve 25 respectively. A pressure drop servo throttle valve (PDSTV) 28 is provided in the flow line 13 to the pilot burners and is open or closed to throttle the flow to the pilot burners in order to maintain a substantially constant pressure drop across the pilot burner flow orifice of the staging valve 25. The valve 25 is controlled by the EECU to divide the flow of fuel from the FMU to the FSU between the pilot burners and the main burners of the engine in a predetermined manner dependent upon the required engine operating conditions to minimise harmful emissions.
Aircraft gas turbine engines, irrespective of whether or not they have staged combustion, can experience an operating condition in which they produce more thrust than is required at that point in the operating cycle of the aircraft. A level of developed thrust in excess of the thrust commanded by the EECU is referred to as “overthrust” and can arise from a number of fault conditions, for example a fault in which the fuel metering valve is moved to a position in which it provides a fuel flow in excess of that commanded by the engine control unit in accordance with the required operating mode of the engine. When the aircraft is on the ground it is generally acceptable for an overthrust event to be terminated by the engine overspeed protection arrangement in which the excessive engine speed is detected and used to operate the TOSSV 22 to close the PRSOV 17 and thus isolate the FSU 12 and the burners from the fuel supply. Such an arrangement accommodates EECU or FMU failure resulting in excessive metering valve opening and shuts down the engine to prevent overthrust increasing to a dangerous extent.
During flight however the situation is somewhat more critical since an overthrust event in one engine of, for example, a twin engine aircraft could, if dealt with by engine shut-down, result in potentially dangerous aircraft yaw conditions. For example, consider a situation in which an aircraft is on final approach to a runway. If one of the two engines experiences an overthrust event the pilot will compensate for the yaw resulting from the overthrust, by use of the aircraft rudder. Should the engine overspeed protection system then operate to close the PRSOV 17 by means of the TOSSV 22 the overthrust will be substantially instantaneously replaced by zero thrust (and possibly engine “flame-out”), at a point when the rudder setting commanded by the pilot is completely opposite to that required to compensate for engine shut down. The aircraft could thus perform a violent, and potentially dangerous yaw manoeuvre in the opposite direction. Accordingly it is considered inappropriate for there to be an overthrust control which shuts the engine down, and clearly the engine speed must not be allowed to increase to a level at which the engine overspeed control will initiate engine shut-down.
Our co-pending United States Patent Application 2003/0019203 A1 discloses a fuel system for a non-staged combustion gas turbine engine in which an additional torque motor operated valve is incorporated into the fuel system, in association with the metering valve, to modify the operation of the metering valve in the event of detection, by the EECU, of an overthrust event.
The provision of overthrust protection in the fuel supply system of a staged combustion engine would necessitate the incorporation of a number of additional control components, and it is an object of the present invention to provide a fuel system for a staged combustion engine in which no additional control valve components are used.